The present invention relates to toners and development systems for toners. More particularly, the present invention relates to electrophotographic toners and developers which have superior electrophotographic stability especially with respect to small charge to mass variability over a wide range of temperatures and relative humidity environmental conditions.
Digital printers and similar products can exhibit undesirable humidity sensitivity and temperature sensitivity which affects the image quality of the image. In more detail, the sensitivity generally takes the form of the developer charge per mass increasing with increasing relative humidity. This particular problem can be seen, for instance, with respect to the initial start up of a copier. When a copier is running, typically the interior of the toning station can be about 20° F. hotter than ambient air, due to various sources of heat including the fuser and the toner station itself. This temperature rise results in effectively lowering the relative humidity seen by the developer to about half of that of the ambient air. Thus, the developer is running at a lower charge per mass value. During an overnight period, when the copier is off, the developer can generally become equilibrated with the higher humidity outside air, but it cannot change its value of charge until it is agitated by starting the use of a copier. What is then observed upon startup is a rapid increase in the charge of the developer and a drop off in copy density.
The developer charge to mass ratio is a primary factor in determining the amount of toner that is transferred from an electrophotographic developer station, for instance, a hard magnetic based carrier electrophotographic developer station, to the image. The high charged toner will generally deposit less toner than a low charged toner at the same exposed film voltage. Variations in the amount of toner transferred to the film results in variations in the reflection print density of the image. This is particularly noticeable when reproducing half-tone or gray scale images. This variability can be the result of various environmental factors, such as temperature and relative humidity, or time dependent charge rise or decay in the developer mixture. This effect means that process control systems or environmental control systems must be designed and built into a printer to compensate for varying developer charge to mass values. Formulating materials not effected by these various factors can reduce complexity and cost to design and build a printer.
Accordingly, there is a need to provide toners and development systems for toners that provide low charge to mass variation with respect to humidity variations or operation over long periods of time. Unless stated otherwise, all wt % are based on the weight of the toner particles.